Major source of Acetyl CoA:

As nothing is mentioned we have to assume question is asking about fed-state. In fed-state, major source of fuel is carbohydrates followed by fats and proteins. Acetyl CoA is derived from Carbohydrates, Fats & Proteins also. So in fed state major source will be Carbohydrates. Glucose forms pyruvate & then it forms Acetyl CoA. So best answer here is Pyruvate.